1. Field of the Invention
The present invention relates, in general, to an apparatus and method for measuring the size of fine nanoparticles present in an aqueous solution as an infinitesimal quantity, and, more particularly, to a scheme that remotely measures the laser-induced breakdown of fine nanoparticles using a probe beam in a non-contact manner, and measures the size of nanoparticles using the frequency distribution curve of the magnitude of a probe beam signal deflected by the laser-induced breakdown. This scheme is a technology that uses a principle by which the path of a probe beam is changed by a laser-induced shock wave accompanying the occurrence of the laser-induced breakdown, and that enables the sizes of nanoparticles to be discriminated by measuring the magnitudes of probe beam signals.
Further, the present invention proposes a method that sets experimental conditions such that the frequency distribution curve of the magnitude of a deflected probe beam signal is a horizontally symmetrical shape, and performs curve fitting on data about the measured magnitude of the deflected probe beam signal to form a normal distribution curve having the shape of a Gaussian function, thus more easily quantifying the peak and full-width at half-maximum of a frequency distribution curve proportional to the diameter of nanoparticles.
2. Description of the Related Art
A technology that has been generally used to measure colloidal nanoparticles present in an aqueous solution is a method of allowing a Continuous Wave (CW) laser beam to be incident on a sample and measuring the intensity of light scattered from the particles. Most turbidimeters and particle size analyzers that are currently commercialized and sold adopt such a light scattering intensity measurement method. When a particle having a size of larger than 100 nm is intended to be measured, it is possible to use a commercial device that adopts the measurement of light scattering intensity. However, when a fine nanoparticle having a size of smaller than 100 nm is intended to be measured using the above commercial device, there is a limitation in that reliable results can be obtained only when the particle density is above several Parts Per Million (ppm). The reason for this is that, as the particle size decreases, light scattering intensity greatly decreases, so that measurement is possible only under the condition that a relatively larger number of particles instead of a large-size particle can contribute to scattering. Therefore, when fine nanoparticles having a density of less than several ppm are intended to be measured, a device having better sensitivity than the commercial device which adopts the measurement of light scattering intensity must be employed.
Accordingly, in order to measure a nanoparticle with high sensitivity, Laser-Induced Breakdown Detection (LIBD) technologies, for example, U.S. Pat. No. 5,316,983 (1994), German Patent No. DE19833339C1 (2000), and Korean Patent No. 10-0820776 (2008) filed by the present applicant, have been proposed. Such an LIBD technology is a technology using the principle by which laser-induced plasma is generated in the focal region of a lens when a pulse laser beam having a temporal width of several nanoseconds is incident on a sample using the lens. Since the energy of a laser beam required to generate laser-induced plasma increases in the order of solid, liquid and gas, only colloidal nanoparticles contained in the aqueous solution are broken down by using the energy of a suitable laser beam, and thus can be generated in the state of laser-induced plasma. When LIBD technology is used, the limits of the detection of the size and density of nanoparticles are about several nm and several Parts Per Trillion (ppt), respectively, and very excellent measurement sensitivity can be realized compared to a commercial device that adopts the measurement of light scattering intensity.
In LIBD technology, in order to detect laser-induced plasma, a laser-induced shock wave or a plasma flash inevitably accompanying the generation of the laser-induced plasma must be measured. For this measurement, a method of firmly attaching a piezoelectric transducer (PZT) to a sample cell and acoustically measuring a laser-induced shock wave, a method of installing a Charge Coupled Device (CCD) camera near a sample cell and optically measuring a flash appearing when laser-induced plasma is generated, and a method of passing a probe beam through a sample cell and optically measuring a laser-induced shock wave, have been independently developed.
In LIBD technology, the size of a particle can be determined under two different experimental conditions.
First, under an experimental condition that the pulse energy of a laser beam changes, the size of a nanoparticle can be determined by measuring threshold energy defined as minimum laser beam pulse energy required to generate laser-induced plasma. This method uses a principle by which, as the size of a particle increases, threshold energy decreases. Generally, threshold energy can be measured by representing breakdown probability by a function of the pulse energy of a laser beam. Breakdown probability is defined as a value obtained by dividing the number of times that laser-induced breakdown occurs by the total number of incident laser beam pulses. For particles having the same size, as the particle density increases, the breakdown probability thereof increases.
Second, under an experimental condition that the pulse energy of a laser beam is fixed, a method using the frequency distribution curve of the magnitude of a PZT signal and a method using the spatial distribution of a plasma flash measured using a CCD camera have been developed.
However, the method of determining the size of a nanoparticle using the frequency distribution curve of the magnitude of a PZT signal is disadvantageous in that it is difficult to obtain reproducible data due to characteristics that a PZT is directly attached to a sample cell to measure a shock wave. That is, the reason for this disadvantage is that, when samples having particles of different sizes are contained in different sample cells, there is a difficulty when the magnitudes of signals are compared with each other by firmly attaching PZTs to a plurality of different sample cells under the same condition.
The method using the spatial distribution of a plasma flash is advantageous in that reproducible data can be obtained compared to the method using the frequency distribution of the magnitude of a PZT signal. However, this method is inconvenient because several application cases are required in that a high-magnification lens array system must be installed near a sample cell in order to record a flash in a camera pixel having a limited size. For example, when elements harmful to human bodies, such as radioactive substances, and ultra-clean water are used as samples, the samples must be located in a special environment isolated from the surroundings (in a glove box or in a cleaning booth), so that it is undesirable to install a high-magnification lens array system occupying a large space near the sample cells.
Therefore, the present applicant proposed in Korean Patent No. 10-0820776 a method of determining the size of a nanoparticle by measuring and analyzing the frequency distribution curve of the magnitude of the probe beam signal using a probe beam which can be remotely measured in a non-contact manner, unlike a method using a PZT or a CCD camera. The method using a probe beam signal is intended to use a principle by which the path of a probe beam is changed by a laser-induced shock wave accompanying the occurrence of laser-induced breakdown. Thanks to the characteristics that the peak of a frequency distribution curve is proportional to the diameter of a nanoparticle, the size of the nanoparticle can be determined. Remotely measuring the size of a nanoparticle contained in an aqueous solution using a probe beam in a non-contact manner is advantageous in that the applicability thereof is much better than that of the conventional method using the frequency distribution of the magnitude of a PZT signal or the spatial distribution of a plasma flash.